Mirnov calibration
Contents
Mirnov calibration
Apparatus
Setup
The power supply S1 acts as a current supply driving 10-15A AC (peak +/-120V) from 100Hz to 10kHz through the Helmholtz coil (L1). Voltage is read across a 10:1 voltage divider (R1&R2) and connected by BNC (J1) to Ch1 of an oscilloscope. Current is read by a Model 110(A?) Pearson Transformer (PT1) on Ch2 of the oscilloscope through the Pearson's BNC connector (J2).
Helmholtz coil
Mirnovs are calibrated on a 3.5" Helmholtz coil. The Helmholtz frame is constructed in G10 with nylon screws and bolts. Using the ideal Helmholtz expression,
with N the number of turns per coil, I the current per winding, and R the Helmholtz radius, and the ideal wire gauge and associated amplifier output parameters can be estimated. The limitations of available power supplies will strongly influence the optimal wire gauge: smaller gauge wire provides more windings per coil, resulting in higher field per ampere but also increasing inductance and therefore required supply voltage per ampere. This essentially means that smaller gauge allows higher field at low frequency, but constrains maximum field at high frequency. Schematics for the Helmholtz form can be found here.
Power Supply
A BOP100-4M power supply is available from the calibration lab as an amplifier; alternatively, a Crown-style Techron7560 audio power amplifier may be borrowed from the electrical engineering department. The BOP is a 400W supply with a 100Vx4A swing; the Techron is rated for 1350W at 4Ω, although its V-I graph suggests the limit of its operational range is roughly 110Vx15A with a mainly inductive load.
Given these power supply specifications, the following magnetic fields and frequencies are accessible, either current limited (C) or voltage limited (V):
| BOP |
Techron | |||||
| Gauge (AWG) |
1kHz (G) |
10kHz (G) |
1kHz (G) |
10kHz (G) | ||
| 20 |
21 (C+V) |
2 (V) |
26 (C+V) |
2.4 (V) | ||
| 18 |
14 (C) |
3.5 (V) |
42 (C) |
4.5 (V) | ||
| 16 |
9 (C) |
6 (V) |
32 (C) |
7 (V) | ||
| 14 |
5.5 (C) |
5.5 (C) |
20 (C) |
12.5 (V) | ||
| 12 |
3.5 (C) |
3.5 (C) |
12.5 (C) |
12.5 (C) | ||
| 10 |
2 (C) |
2 (C) |
7.5 (C) |
7.5 (C) | ||
It should be noted that the BOP may not like driving an almost completely inductive load (power fraction ~0.08% at 10kHz), whereas the Techron is known to be tolerant to inductive loads.
The supply will drive the Helmholtz through a Pearson transformer. An oscilloscope will be connected to the Pearson, and across a high-impedance (~10kΩ+) 10:1 voltage divider to measure the coil current and voltage, respectively.
There is also a Crown XLS 2502 audio amplifier available for use as a power amplifier. As of July 7th, it has not yet been thoroughly tested, but it has been used to drive the Helmholtz at more than 15A (~17G) at low frequencies (~1kHz and lower).
Gaussmeter
A DC supply or BOP with a high current limit (ideally 10+ A) can first be used with a gaussmeter to measure the Helmholtz coil's true G/A, so that the current measurement will serve as a field measurement.
As of July 7th, the highest-current DC calibration test (at 10.2A DC) found the Helmholtz coil produces 1.15±0.03 G/A.
Procedure
Each TA Mirnov was calibrated at 100Hz, 300Hz, 1kHz, 3kHz, 5kHz, and 10kHz.
Each old "gap" Mirnov was calibrated at 1kHz and 5kHz only to estimate number of turns.
Each SES Mirnov (the 6 best of the above "gap" Mirnov pairs, plus one additional spare Mirnov pair) was calibrated at 100Hz, 300Hz, 1kHz, 3kHz, and 10kHz.
Results
Plots from the TA Mirnov calibration can be found here. Plots from the calibration of old "gap" Mirnovs to be used for shell eddy sensors Mirnov calibration can be found here.
